workflows/ci_docs: lots of goodies (#13809)

* workflows/ci_docs: publish documentation to Github Pages

This should be a complete replacement for our current Travis CI setup.

* workflows/ci_docs: run docgen upon modification to the css on push

So that any changes regarding the stylesheets would be reflected on the
published docs.

* workflows/ci_docs: build the compiler in release mode

* workflows/ci_docs: set branch name for the generated docs

This makes the "Source" links work correctly.

* workflows/ci_docs: run docgen on windows and osx too

Only deploy the Linux-generated version.

* workflows/ci_docs: cache csources compiler

This should cut the time spent building csources, which is about 1-2mins
depending on OS.

.github/workflows/ci_docs.yml

@@ -1,36 +1,105 @@
 name: Nim Docs CI
 on:
+  push:
+    # Run only on changes on these files
+    paths:
+      - 'lib/**.nim'
+      - 'doc/**.rst'
+      - 'doc/nimdoc.css'
+
   pull_request:
     # Run only on changes on these files
     paths:
-      - 'lib/*.nim'
-      - 'doc/*.rst'
+      - 'lib/**.nim'
+      - 'doc/**.rst'
 
 jobs:
   build:
-    name: 'Docs builder'
-    runs-on: ubuntu-18.04
+    strategy:
+      fail-fast: false
+      matrix:
+        target: [linux, windows, osx]
+        include:
+          - target: linux
+            os: ubuntu-18.04
+          - target: windows
+            os: windows-2019
+          - target: osx
+            os: macos-10.15
+
+    name: ${{ matrix.target }}
+    runs-on: ${{ matrix.os }}
+
     steps:
       - name: 'Checkout'
         uses: actions/checkout@v2
 
-      - name: 'Checkout csources'
-        uses: actions/checkout@v2
-        with:
-          repository: nim-lang/csources
-          path: csources
+      - name: 'Install build dependencies (macOS)'
+        if: runner.os == 'macOS'
+        run: brew install make
+
+      - name: 'Install build dependencies (Windows)'
+        if: runner.os == 'Windows'
+        shell: bash
+        run: |
+          mkdir dist
+          curl -L https://nim-lang.org/download/mingw64.7z -o dist/mingw64.7z
+          curl -L https://nim-lang.org/download/dlls.zip -o dist/dlls.zip
+          7z x dist/mingw64.7z -odist
+          7z x dist/dlls.zip -obin
+          echo "::add-path::${{ github.workspace }}/dist/mingw64/bin"
 
       - name: 'Add build binaries to PATH'
         shell: bash
         run: echo "::add-path::${{ github.workspace }}/bin"
 
+      - name: 'Get current csources version'
+        id: csources-version
+        shell: bash
+        run: |
+          sha=$(git ls-remote https://github.com/nim-lang/csources master | cut -f 1)
+          echo "::set-output name=sha::$sha"
+
+      - name: 'Get prebuilt csources from cache'
+        id: csources-cache
+        uses: actions/cache@v1
+        with:
+          path: bin
+          key: '${{ matrix.os }}-${{ steps.csources-version.outputs.sha }}'
+
+      - name: 'Checkout csources'
+        if: steps.csources-cache.outputs.cache-hit != 'true'
+        uses: actions/checkout@v2
+        with:
+          repository: nim-lang/csources
+          path: csources
+
       - name: 'Build 1-stage compiler from csources'
         shell: bash
         run: |
-          ncpu=$(nproc)
-          [[ -z "$ncpu" || $ncpu -le 0 ]] && ncpu=1
+          ext=
+          [[ '${{ runner.os }}' == 'Windows' ]] && ext=.exe
+          if [[ ! -x bin/nim-csources$ext ]]; then
+            ncpu=
+            case '${{ runner.os }}' in
+            'Linux')
+              ncpu=$(nproc)
+              ;;
+            'macOS')
+              ncpu=$(sysctl -n hw.ncpu)
+              ;;
+            'Windows')
+              ncpu=$NUMBER_OF_PROCESSORS
+              ;;
+            esac
+            [[ -z "$ncpu" || $ncpu -le 0 ]] && ncpu=1
 
-          make -C csources -j $ncpu CC=gcc ucpu='amd64'
+            make -C csources -j $ncpu CC=gcc
+            cp bin/nim{,-csources}$ext
+          else
+            echo 'Cache hit, using prebuilt csources'
+            cp bin/nim{-csources,}$ext
+          fi
 
       - name: 'Build koch'
         shell: bash
@@ -38,8 +107,21 @@ jobs:
 
       - name: 'Build the real compiler'
         shell: bash
-        run: ./koch boot
+        run: ./koch boot -d:release
 
       - name: 'Build documentation'
         shell: bash
-        run: ./koch doc
+        run: ./koch doc --git.commit:devel
+
+      - name: 'Prepare documentation for deployment'
+        if: github.event_name == 'push' && github.ref == 'devel' && matrix.target == 'linux'
+        shell: bash
+        run: cp -f doc/html/{overview,index}.html
+
+      - name: 'Publish documentation to Github Pages'
+        if: github.event_name == 'push' && github.ref == 'devel' && matrix.target == 'linux'
+        uses: crazy-max/ghaction-github-pages@v1
+        with:
+          build-dir: doc/html
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}